Improvement of covering power of silver halide emulsions makes it possible to save silver weight necessary for attaining a given optical density, and hence is a very important subject for those who manufacture emulsions. For this reason, a number of proposals have been hitherto made. For example, U.S. Pat. Nos. 4,411,986, 4,434,226, 4,413,053, etc. disclose that the covering power of developed silver can be improved by the use of tabular grains obtained by making small the thickness of silver halide grains to have a high aspect ratio. Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 108526/1983 also discloses that a high sensitivity and a high covering power can be achieved by the use of what is called epitaxial grains formed by providing another silver salt on such tabular silver halide grains having a high aspect ratio. Besides the foregoing, it is also well known from a number of reports that tabular silver halide grains have a higher covering power than usual grains.
The tabular silver halide grains, however, impart without exception a yellowish tone to the image silver formed by photographic processing. This may consequently offend viewers when silver images are viewed as in, for example, X-ray light-sensitive photographic materials. This is due to the fact that the tone of images depends on the size of developed silver and the developed silver of a grain having a small grain thickness diffuses blue-color components to give a yellowish tone.
As a tone modifier of developed silver, a number of compounds are conventionally known, as exemplified by specific mercapto compounds, heterocyclics or hydrazine compounds. These conventional compounds, however, may inhibit the aforesaid high covering power that is characteristic of the tabular grain emulsions, and also may cause desensitization.